Pneumatic tyre

Abstract: A pneumatic tyre having an improved tread profile capable of improving wet grip and running noise without sacrificing dry grip. The tread portion (T) is provided with a pair of circumferential grooves (7) to divide the tread portion into a central portion (9) and a pair of shoulder portions (8). The contour of the central portion (9) comprises a central face (9b) defined by a convex curve and a pair of side faces (9a) each defined by a concave curve. Each of the side faces (9a) extends axially outwardly and radially inwardly from one axial edge (F) of the central face towards the bottom of the adjacent circumferential groove (7). The side faces (9a) intersect the central face (9b) to form an angle of less than 180 degrees. In a standard condition in which the tyre is mounted on a standard rim, inflated to 70% of the maximum air pressure, and loaded with 70% of the maximum load, the ground contacting width (CW) of the central portion (9) is not greater than the axial width (SW) between the axial edges (F) of the central face.

Description: The present invention relates to a pneumatic tyre having an improved tread profile capable of improving the wet grip performance and tyre noise while maintaining dry grip performance. In recent years, as the expressway network is developed and vehicle performance is remarkably improved, a tread pattern which includes circumferentially extending straight or generally straight wide main grooves, has been widely used for its superior drainage performance. In the ground contacting patch, air tubes are formed between the ground and circumferential grooves. As a result, air existing in the tube is excited by impact sound, vibrations, pulsative air flow and the like, and air resonates at a certain frequency (about 800 to 1250 Hz) to generate so called air resonance noise. If the volume of circumferential groove and/or the number of the circumferential grooves are decreased, such resonance noise can be reduced, but wet performances are greatly decreased. If the volume and number of the circumferential grooves are increased, wet performances such as wet grip are improved, but the dry grip performance and steering stability are liable to decrease. In order to solve these problems, in Japanese patent application laid open No. JP-A-6-127215 (appln. No.4-302955), a pneumatic tyre having the novel tread profile shown in Fig.11 has been proposed, wherein the tread portion is divided by two circumferential grooves (e) into a central portion (j) and two shoulder portions (k). The central portion (j) has a contour (h) defined by a convex curve (g) extending continuously between the edges (f) of the bottoms of the grooves (e), whereby the drainage is improved to prevent the occurrence of the aquaplane phenomenon and the wet grip performance is improved. In this proposal, however, as the contour (h) is defined by a single convex curve, as tread wear progresses, the groove width of the circumferential groove (e) is greatly decreased, in other words, decrease in the groove volume is accelerated, and wet performances such as the resistance to aquaplaning and wet grip performance are greatly decreased. Another tyre having a convex shaped central tread region between two circumferentially extending grooves is known from JP-A-5147407. This tyre exhibits reduced steering force and improved handling without loss of stability. It is therefore, an object of the present invention to provide a pneumatic tyre in which wet performances improved by the above-mentioned proposition are maintained, even as tread wear progresses, without sacrificing other performances such as dry grip, running noise and the like. According to the present invention, a pneumatic tyre comprising a tread portion, provided with a pair of circumferential grooves extending substantially continuously in the circumferential direction of the tyre, one circumferential groove being disposed on each side of the tyre equator to divide the tread portion into a central portion between the circumferential grooves and a pair of shoulder portions axially outwards of the circumferential grooves, the central portion having a contour comprising a central face defined by a continuous convex curve having axial edges and a pair of side faces, each of the side faces extending axially outwardly and radially inwardly from each axial edge of the central face towards the bottom of the adjacent circumferential groove, the side faces intersecting the convex central face at the axial edges to form an angle of less than 180 degrees, and in a standard condition in which the tyre is mounted on a standard rim, inflated to 70% of the maximum air pressure, and loaded with 70% of the maximum load, the ground contacting width of the central portion is not greater than the axial width between the axial edges of the central face, characterised in that each of the pair of side faces is defined by a continuous concave curve and under the standard condition, the axial width of each of the circumferential grooves in the new tyre state is in the range of from 20% to 30% of the tread width. Here, the standard rim is the rim officially approved for the tyre by, for example JATMA (Japan), TRA (USA), ETRTO (Europe) and the like. Also the maximum air pressure and maximum tyre load are those officially specified by the same association or organisation. As a result, the volume of the circumferential grooves is increased to improve drainage, and aquaplaning phenomenon is prevented to improve the wet grip performance. Further, a sufficient ground contacting area can be obtained to maintain the dry grip performance. Furthermore, as can be seen in the foot print of Fig. 8, the circumferential groove changes its width in the longitudinal direction such that the width is a minimum in the centre and becomes wider towards both ends. That is, the above-mentioned air tube changes its sectional area and shape. Accordingly, the resonance mode becomes complex. As a result, resonance hardly occurs and the air resonance noise is reduced. Even if tread wear makes progress, the convex side faces decrease change in the width of the circumferential groove, which prevents the resistance to aquaplaning performance from being decreased. An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings in which: Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is a partial plan view showing the tread pattern; Fig.3 is an enlarged partial cross sectional view of the tread; Fig.4 is a graph showing the relationship between running noise and groove width; Fig.5 is a graph showing the frequency spectrum; Fig.6 is a graph showing the relationship between cornering power and total groove width; Fig.7 is a graph showing the relationship between the critical speed for aquaplaning phenomenon and total groove width; Fig.8 is a schematic view showing the ground contacting area of the tread (foot print); Fig.9 is a cross sectional view of the tread portion of a prior art tyre; Fig.10 is a partial plan view of the prior art tyre; and Fig.11 is a cross sectional view showing the tread profile disclosed in Japanese patent application laid open No. JP-A-6-127215. In Fig.1, a tyre 1 according to the present invention is a passenger radial tyre having a low aspect ratio mounted on a standard rim R and inflated to 70% of the maximum air pressure. The aspect ratio, which is a ratio of the tyre section height to the tyre section width, is 0.4 to 0.6. The tyre 1 comprises a tread portion (T), a pair of axially spaced bead portions (B), a pair of sidewall portions (S) extending between the tread edges (TE) and the bead portions (B), a pair of bead cores 2 one disposed in each of the bead portions (B), a carcass 3 extending between the bead portions (B), and a belt 4 disposed radially outside the carcass 3 and inside the tread portion T. The carcass 3 comprises at least one ply of radially arranged cords extending between the bead portions and turned up around the bead cores 2 in the bead portion (B) from the axially inside to outside of the tyre. For the carcass cords, organic fibre cords, e.g. polyester, nylon, rayon or the like are preferably used in the case of a passenger tyre. Between each turnup portion and the main portion thereof, a bead apex 6 made of hard rubber extending radially outwardly from the bead core 2 is disposed to increase the bead rigidity. The belt 4 in this embodiment comprises crossed plies, each made of high modulus cords, e.g. steel, aromatic polyamide or the like, laid at an angle of from 15 to 30 degrees with respect to the tyre circumferential direction. The tread portion (T) is provided on each side of the tyre equator CL with a pair of circumferential grooves 7, each extending substantially continuously in the circumferential direction of the tyre, whereby the tread portion (T) is divided into a central portion 9 between the circumferential grooves 7 and 7 and a pair of shoulder portions 8 axially outwards of the circumferential grooves 7 and 7. In this embodiment, the circumferential grooves 7 are straight. However, in order to provide more traction, it is possible to use a zigzag configuration in the groove sidewall 8a. Preferably, each circumferential groove 7 is disposed in the centre between the tyre equator CL and the tread edge TE or the edge of the tread ground contacting width TW. In a meridian section of the tyre, i.e., a tyre section including the tyre axis, the contour of the shoulder portion 8 is defined by a top face 8b and an axially inner side face 8a. The side face 8a extends radially outwardly from the axially outer edge of the bottom 7a of each circumferential groove 7 in a non-arcuate fashion, for example straight, inclined at an angle (alpha) of 0 to 40 degrees, preferably 5 to 25 degrees with respect to a radial line X, so as to form an edge corner (a) between the inner side face 8a and the top face (8b). The top face 8b extends axially outwardly from the edge corner (a), and the radius of curvature R3 thereof is preferably not less than 3 times the tread width TW of the overall ground contacting width of the tread portion (T). The centre of the radius of curvature R3 is disposed on the tyre equator CL. If the radius R3 is small, the steering stability during cornering and dry grip performance are decreased. A larger radius is more preferable, and the upper limit is infinite, that is, the top face can be straight parallel to the tyre axis. As the edged corner (a) is provided in the shoulder portion 8 whose ground pressure is high, the cornering power is increased to maintain the dry grip performance. The contour of the central portion 9 is defined by a pair of concave side faces 9a and 9a and a convex central face 9b therebetween. Each concave side face 9a extends axially inwardly and radially outwardly from the axially inner edge of the bottom 7a of each circumferential groove 7. The convex central face 9b extends between axially inner edges (F) of the side faces 9a and 9a. The central face 9b is defined by an arc having a radius of curvature R1. The radius R1 is smaller than the radius of curvature R3 of the shoulder portions 8, and preferably in the range of from 0.5 to 1.5 times the tread width TW. If R1 is less than 0.5 times TW, the ground contacting width CW of the central portion 9 decreases, and the dry grip performance is poorer. If R1 is greater than 1.5 times TW, the drainage becomes insufficient and the wet grip is decreased. In this embodiment, the central face 9b has a single radius R1, and the centre is disposed on the tyre equator CL. The above-mentioned side face 9a has a radius of curvature R2. The radius of curvature R2 is preferably in the range of from 0.05 to 0.5 times the tread width TW. If R2 is less than 0.05 times TW, the rigidity of the central portion 9 is liable to decrease. If R2 is more than 0.5 times TW, the decrease in the circumferential groove width due to tread wear is liable to become greater. Each side face 9a intersects the central face 9b at the axial edge (F) to form an angle of less than 180 degrees, preferably 120 ±15 degrees, whereby an edged corner is formed. The height of the central portion 9 is such that the convex central face 9b substantially inscribes an imaginary line 10 which can be drawn, smoothly connecting the top faces 8b of the shoulder portions 8 with each other. The imaginary line 10 is a straight line or a convex line having a single radius of curvature extending between the edges (a) of the shoulder portions 8 and being tangential to the top face 8b at the edge (a). Here, the phrase "substantially inscribe" means that the distance (L) between the imaginary line 10 and the central face 9b at the tyre equator CL is within the range of not more than 2% of the tread width TW. If the distance (L) is not less than 2%, the ground pressure difference between the central portion and the shoulder portions becomes large, and the grip performance and wear resistance are deteriorated. In order to maintain dry grip, wear resistance, steering stability and the like, in the above-mentioned standard condition, the axial width SW of the central face 9b between the edged corners (F) is set in the range of from 5 to 40 %, preferably 15 to 35% of the tread width TW. In this embodiment, further, the ground contacting width CW of the central portion 9 is the same as the width SW of the central face 9b. In other words, the whole width between the edged corners (F) contacts with ground. However, it is possible that the ground contacting width CW is smaller that the width SW. In other words, the edged corners (F) are located axially outward of the edges of the ground contacting width CW. As shown in Fig.3, if the side face 9a is defined by a convex curve as shown by the broken line 9c, which corresponds to the Fig.11 contour, when the tread wear progresses to for example 50% (as shown by chain line M with two dots), the decrease GWO-GW2 in the width of the circumferential groove becomes very large, and the circumferential groove volume becomes very small. Therefore, aquaplaning performance is greatly deteriorated. In the present invention, however, by the provision of the concave side faces 9a, the groove width decrease GWO-GW1 in the 50% wear state becomes very small, and the volume of the circumferential groove is increased to improve the drainage, and the aquaplaning phenomenon is prevented. Here, "50% wear" means that the tread portion is worn and the circumferential groove 7 has decreased in depth to 50% of the depth in the new tyre state. The groove width is the minimum axially distance measured from the edge of the central portion 9 to the edge of the shoulder portion 8 in the foot print obtained under a 70% load state in which the tyre is mounted on the standard rim and inflated to 70% of the maximum pressure and loaded with 70 % of the maximum tyre load. It is preferable that the groove width GW1 in the 50% wear state is in the range of from 0.84 to 0.92 times the groove width GWO in the new state. If GW1 is less than 0.84 times GW0, the aquaplaning performance is liable to be deteriorated. If GW1 is more than 0.92 times GW0, it becomes difficult to provide a necessary rigidity for the central portion 9. Further, to reduce the air resonance noise, the width GWO of the circumferential groove 7 is preferably set in the range of 20 to 30 % of the tread width TW. Fig.4 shows the results of a noise test, wherein the running noise was measured, changing the groove width GWO, maintaining the groove depth constant. The test tyres (size 205/55R15) were provided in the tread portion with a pair of circumferential grooves having a U-shaped sectional shape. According to the "Test Procedure for Tyre Noise" specified in Japanese JASO-C606, a 2000cc passenger car provided with test tyres was coasted for a 50 metre distance at a speed of 60 km/h in a straight test course, and the maximum noise sound level was measured with a microphone set at 1.2 metre height from the road surface and 7.5 metre sideways from the running centre line in the midpoint of the course. As shown in Fig.4, the noise became maximum when the ratio GW/TW of the groove width GW to the tread width TW was 13%, and it is preferable that the ratio is greater than 15%, more preferably greater than 20%. Fig.5 shows the frequency spectrum of the noise when the groove width ratio was 13% and 27%. When the ratio was 27%, the peak noise level at about 1kHz was remarkably decreased in comparison with that of 13%. Fig.6 shows the results of a cornering power test, in which the cornering power was measured with an indoor drum tester, changing the total width of the circumferential grooves 7. The test tyres used were the tyre shown in Figs.1 and 2 (the total groove width = GWOx2) and a conventional tyre shown in Figs.9 and 10 having four circumferential grooves G (the total groove width = GWx4), which had the same tyre size. As shown in Fig.6, from the test results, it was confirmed that the cornering power of the example tyre is greater than that of the conventional tyre. However, when the total groove width exceeds 50% of TW, the cornering power is greatly decreased. Further, changing the total width of the circumferential grooves, the critical speed for aquaplaning phenomenon was measured. The results are shown in Fig.7. From the test results, it was confirmed that when the total groove width is greater than about 25 % of TW, the critical speed of the example tyre is higher than that of the conventional tyre. Therefore, it is preferable that the width GW0 of each circumferential groove 7 is not less than 15% of the tread width TW, and the total of the widths GW0 of all the circumferential grooves 7 is in the range of from 30 to 50% of the tread width TW. Further, the depth of each of the circumferential grooves 7 is preferably in the range of from 4 to 8 % of the tread width TW. In the embodiment shown in Fig.1, the side face 9a and the central faces 9b are defined by a single radius arc. However, a multi radius arc, an ellipse, or a curve closely resembling an ellipse may be used. In the present invention, the shoulder portions 8 and central portion 9 can be provided with axially extending grooves to improve the wet grip performance and road grip. Fig.2 shows an example of the tread pattern including such axial grooves, wherein each shoulder portion 8 is provided with axial grooves 15 and 16. Each of the axial grooves 16 extends axially outwardly from one of the circumferential grooves 7 over the tread edge TE, with both the axial ends opened. Each of the axial grooves 15 extends axially outwardly from a position axially outward of the circumferential groove 7 to the tread edge TE, with both the axial ends closed. The axial grooves 15 and the axial grooves 16 are arranged alternately in the tyre circumferential direction, whereby the wet grip performance can be improved without decreasing the rigidity of the shoulder portion 8. The central portion 9 is provided with axial grooves 12. Each of the axial grooves 12 has an axial inner end positioned near the equator CL and an axially outer end opened to one of the circumferential grooves 7. No axial groove is provided near the tyre equator CL, so that the rigidity of the central portion is maintained, and steering stability is thus provided. As shown in Fig.1, the axial grooves 11(15 and 16) and 12 has a groove bottom 11a and 12a being substantially parallel with the belt 4. The axially inner closed ends 11b,12b of the axial grooves 11,12 are parallel to the tyre equator CL or inclined at a small angle (beta) of less than 15 degrees with respect to a radial line (Y). Therefore, when tread wear progresses, the length of the axial groove is hardly decreased, and the wet grip performance is maintained. Test tyres of size 205/55R15 having specifications shown in Table 1 were tested for aquaplaning performance and running noise. Aquaplaning Performance Test A test car provided on all four wheels with test tyres was run on a wet asphalt road with a water depth of 5mm around a 100 metre radius circle at a speed of 70 kilometre/hour, and the maximum lateral-G was measured. The test results are shown in Table 1, wherein the results are indicated by an index based on that the conventional tyre is 100. The larger the index, the higher the resistance to aquaplaning. From the test results, it was confirmed that the example tyres according to the invention are superior in the aquaplaning performance to the reference tyres in both the new tyre state and 50% wear state. The running noise was also improved without sacrificing aquaplaning performance. As explained above, in the pneumatic tyre according to the present invention, even as tread wear progresses, the wet grip performance is maintained at an improved level, without deteriorating the dry grip performance and air resonance noise. Tyre Ex.1 Ex.2 Ex.3 Ref.1 Ref.2 Fig.1 Fig.1 Fig.1 Fig.9 Fig.11 Tread width TW (mm) 168 168 168 168 168 Central portion Radius R1 (mm) 85 85 85 - 85 R2 (mm) 25 15/160 40 - - Ground contacting (mm) width 38 38 38 - 38 Shoulder portion Radius R3 (mm) 900 900 900 900 900 Circumferential groove Number 2 2 2 4 2 Depth (mm) 8.4 8.4 8.4 8.4 8.4 Width New GWO (mm) 38 38 38 9/9.5 38 50% wear GW1 (mm) 32 34 29 - - 50% wear GW2 (mm) - - - - 27 Width change from (%) new to 50% wear 16 11 24 - 29 Test Result Aquaplane New 142 142 141 100 141 50% wear 99 103 94 77 90 50% wear/new (%) 70 73 67 77 64 Running noise New dB(A) -2.5 -2.4 -2.5 0 -2.5

Claims: A pneumatic tyre comprising a tread portion (T), provided with a pair of circumferential grooves (7) extending substantially continuously in the circumferential direction of the tyre, one circumferential groove (7) being disposed on each side of the tyre equator (CL) to divide the tread portion (T) into a central portion (9) between the circumferential grooves (7) and a pair of shoulder portions (8) axially outwards of the circumferential grooves (7), the central portion (9) having a contour comprising a central face (9b) defined by a continuous convex curve having axial edges (F) and a pair of side faces (9a), each of the side faces (9a) extending axially outwardly and radially inwardly from each axial edge (F) of the central face (9b) towards the bottom of the adjacent circumferential groove (7), the side faces (9a) intersecting the convex central face (9b) at the axial edges (F) to form an angle of Less than 180 degrees, and in a standard condition in which the tyre is mounted on a standard rim, inflated to 70% of the maximum air pressure, and loaded with 70% of the maximum load, the ground contacting width (CW) of the central portion (9) is not greater than the axial width (SW) between the axial edges (F) of the central face, characterised in that each of the pair of side faces (9a) is defined by a continuous concave curve and under the standard condition, the axial width (GWO) of each of the circumferential grooves (7) in the new tyre state is in the range of from 20% to 30% of the tread width (TW). A pneumatic tyre according to claim 1, characterised in that the central face (9b) has a radius (R1) in the range of from 0.5 to 1.5 times the tread width (TW). A pneumatic tyre according to claim 1, characterised in that the central face (9b) has a single radius (R1) in the range of from 0.5 to 1.5 times the tread width (TW). A pneumatic tyre according to claim 1, 2 or 3, characterised in that each of the side faces (9a) has a radius (R2) in the range of from 0.05 to 0.5 times the tread width (TW). A pneumatic tyre according to claim 1, 2 or 3, characterised in that each of the side faces (9a) has a single radius (R2) in the range of from 0.05 to 0.5 times the tread width (TW). A pneumatic tyre according to any of claims 1 to 5, characterised in that the angle between the central face (9b) and each side face (9a) at their intersect (F) is in the range of from 105 to 135 degrees. A pneumatic tyre according to any of claims 1 to 6, characterised in that the axial width (GWO) of each of the circumferential grooves (7) under the standard condition is such that the width in a 50% wear state is not less than 84% of the width in the new tyre state, where the 50% wear state is when tread wear has progressed to 50 % of the new tyre depth of the circumferential grooves (7).

Inventors: IWASAKI NAOAKI, IWASAKI, NAOAKI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1994
Main classification: B60C11
IPC codes: B60C11, G10K11